RuneScape is a Free to Play, Browser Based Role-Playing MMO Game using Java and doesn’t require any downloads at all.
"RuneScape is one of the largest, most expansive online fantasy games on the web today. It has over 175 million registered accounts and a loyal following that continues to make incredible use of its bravery, intuition, cunning and wit to explore the game world, discovering adventure, battling rival clans and conquering a massive online world.
RuneScape continues to expand all the time, but membership can open up whole new areas to conquer. A RuneScape subscription will grant you access to a world over three times larger than that found in the free game, with more than 150 additional quests to test your abilities. Subscribers will also be able to learn and practice nine new skills, and will have access to 20 minigames that are available to members only."